SAML _security/saml/authenticate API should return realm name used for authentication

[azasypkin]

Currently response of _security/saml/authenticate includes only username, access_token, refresh_token and expires_in. That means that if consumer doesn't provide an optional realm parameter they don't know what realm was used to perform authentication unless they send a subsequent request to /_security/_authenticate and read authentication_realm field.

It's not critical, but rather nice to have since Kibana relies on the realm name for various use cases and it'd be ideal to avoid additional /_security/_authenticate call right after _security/saml/authenticate.

/cc @jkakavas

[elasticmachine]

Pinging @elastic/es-security (:Security/Authentication)